1. Field of the Invention
The present invention relates to a nonreciprocal circuit device, such as an isolator or a circulator, for use in a high-frequency band such as the microwave band, to a communication device comprising the nonreciprocal circuit device, and to a method of manufacturing the nonreciprocal circuit device.
2. Description of the Related Art
Generally, nonreciprocal circuit devices, such as isolators or circulators, used in mobile communication devices such as portable phones, have a function of allowing signals to pass through in a predetermined transmission direction and of preventing the transmission in the reverse direction.
This type of nonreciprocal circuit device is constructed by housing a permanent magnet, a magnetic material (ferrite) to which a DC magnetic-field is applied by the permanent magnet, and component members such as a plurality of center conductors arranged on this magnetic material, inside a metal case. The metal case is formed by bonding an upper metal case made of a magnetic-material metal to a lower metal case made of a magnetic-material metal.
Nonreciprocal circuit devices, in which an upper metal case and a lower metal case which form this metal case are bonded by resistance welding, are proposed in, for example, Japanese Unexamined Patent Application Publication Nos. 10-107513 and 10-276011. In these publications, in the nonreciprocal circuit devices, an upper metal case and a lower metal case are resistance-welded with their mutually bonded surfaces in surface contact.
As described in Japanese Unexamined Patent Application Publication No. 10-107513, as a result of bonding the two metal cases by resistance welding, the problem of a defective connection caused by remelting of solder, which occurs when the metal cases are bonded by soldering, can be reduced. Also, it is described in Japanese Unexamined Patent Application Publication No. 10-276011 that, as a result of bonding the metal cases by resistance welding, the magnetic resistance of the bonded portions of the metal cases can be reduced in comparison with conventional bonding by soldering and crimping, and the external magnetic-field can be made effectively strong.
However, there is a problem in the above-described conventional nonreciprocal circuit devices, in which the resistance welding of the upper and lower metal cases is performed with their mutually bonded surfaces in surface contact. Stable and reliable resistance welding cannot be performed due to processing variations in their bonded surfaces, and variations in the component members, etc., incorporated in the nonreciprocal circuit device. Thus, the bonding strength and the electrical characteristics (insertion loss, isolation, etc.) vary greatly, and as a result, a desired bonding strength and desired electrical characteristics cannot be obtained. That is, in a conventional nonreciprocal circuit device, since the two metal cases are in surface contact at the bonded surfaces, the contact portions, the contact state, and the contact area are not stable. Also, variations in the bonding process are large and the bonding strength is decreased under predetermined welding conditions (a fixed welding current, and a fixed current flowing time). Furthermore, since the portions which are welded and the bonding strength become unstable, the electrical/magnetic circuit characteristics change. For example, the magnetic resistance in the bonded portion may be increased, or the electrical characteristics may vary greatly or the electrical characteristics may be degraded.